This invention relates to free-piston Stirling engines (FPSE) and more particularly relates to an improvement which causes the engine to be automatically depowered in the event that the engine load, as seen by the engine at its output, changes in a manner that the engine would become unstable, for example because of a failure of the engine's controller or wiring to the controller. This depowering prevents an increase of piston amplitude of reciprocation that would otherwise cause a runaway amplitude increase resulting in the piston having engine-damaging collisions with other internal engine components.
A problem with free-piston Stirling engines is that historically they have not been tolerant to loss of load. A kinematic Stirling machine that is adequately designed will, when its load is removed or reduced, often just run at a higher speed and the machine's internal heat exchanger pumping losses consume the power produced. However a FPSE is a resonant machine and so, if unloaded, the frequency will not change significantly. Instead, the piston and displacer will over-stroke and collide with physical structures within the engine and with each other. The problem is made worse because the power increases not only with amplitude but also because of the resulting discontinuous motions resulting from collisions. The collisions often lead to failure of internal components and to the generation of debris which can lead to engine failure. The purpose of the invention is to provide a FPSE which is tolerant to loss of engine load because such collisions and damage are prevented by the invention if the engine's load is reduced or becomes zero.
FIG. 1 is a diagrammatic illustration of a beta type free-piston Stirling engine that embodies the invention. However, many of the engine's structural features that are symbolically illustrated in FIG. 1 are known in the prior art. Therefore, those features that an embodiment of the invention has in common with the prior art are described in this “Background of the Invention” section. The distinguishing features of the invention are then described in the other sections.
Referring to FIG. 1, in a Stirling engine a working gas is confined in a working space 8 comprised of a heat accepting expansion space 10, an opposite heat rejecting compression space 12 and a working gas flow path between the expansion space 10 and the compression space 12. The working gas flow path includes, in series fluid connection, a heat acceptor 14, which transfers externally applied heat into the working gas, a heat rejecter 16, which transfers heat out of the working gas, and an interposed regenerator 18. The flow path also includes a heat rejecter cylinder port 20 through an engine cylinder 22 at the cylinder's compression space 12 and a heat acceptor cylinder port 24 at the open end of the engine cylinder 22 at the cylinder's expansion space 10. The heat acceptor 14, heat rejecter 16 and regenerator 18 are formed annularly to surround the engine cylinder 22. The heat rejecter cylinder port 20 consists of several such ports located at intervals that are spaced annularly around the cylinder and in common fluid communication. Heat is applied to the heat acceptor 14 and commonly to the entire head end 26 of the engine, such as by a gas flame or the application of concentrated solar energy. Heat is removed from the heat rejecter 16 by an external heat exchanger (not shown) that transfers the heat to the coolant of a cooling system.
Reciprocating motion of the piston 28 and a displacer 30 cause the working gas to be alternately heated and cooled and alternately expanded and compressed in order to do work on the piston 28 that reciprocates in the cylinder 22. The piston 28 has a sidewall 32 that engages and slides along the cylinder 22 and the sidewall has an inward end 34. The terms “in”, “inward”, “out” and “outward” are used as a terminology convention to describe the opposite axial directions of motion of engine components including the piston 28 and the displacer 30. The terms “in” and “inward” indicate a direction or position toward or nearer the working space 8, which includes the compression space 12 part of the working space 8. The terms “out” and “outward” indicate a direction or position away from or farther from the working space 8. The piston 28 also has an annular cutout or relieved portion to form a central cap or boss 36 that is unrelated to the invention. Its purpose is to occupy a volume of the compression space 12 which would otherwise be an unswept volume.
The displacer 30 of a beta type Stirling engine typically reciprocates in the same cylinder 22. The displacer 30 is connected through a displacer connecting rod 38 to a planar spring 40 that is mounted to a casing 42. The casing 42 surrounds a relatively large volume back space 43 and also contains working gas. The reciprocating mass of the piston 28, the reciprocating mass of the displacer 30 and its connecting rod acting upon the planar spring 40 and the resiliently compressible and expansible working gas together form a resonant system which has been called a thermal oscillator.
The reciprocating displacer 30 cyclically shuttles the working gas between the compression space 12 and the expansion space 10 through the heat accepter 14, the regenerator 18 and the heat rejecter 16. This shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space 10, and gas that is flowing into or out of the expansion space 10 through the heat accepter 14 accepts heat from surrounding surfaces. Gas that is in the compression space 12 and gas that is flowing into or out of the compression space 12 through the heat rejecter 16 rejects heat to surrounding surfaces. The rejected heat is ordinarily transferred away by the cooling system. The gas pressure is essentially the same in both spaces 10 and 12 at any instant of time because the spaces 10 and 12 are interconnected through the working gas flow path between the expansion space 10 and the compression space 12 and that flow path has a relatively low flow resistance. However, the pressure of the working gas in the working space 8 as a whole varies cyclically and periodically. The periodic increase and decrease of the pressure of the working gas in the working space 8 drive both the piston 28 and the displacer 30 in reciprocation. The periodic pressure variations are caused by the resultant of two components that are out of phase with each other. The first component is the alternating net heating and cooling of the working gas in the workspace. When a majority of the working gas is in the compression space 12, there is a net heat rejection from the working gas and the first component of gas pressure variation decreases. When a majority of the working gas is in the expansion space 10, there is a net heat acceptance into the working gas and the first component of gas pressure variation increases. The second component of gas pressure variation is the result of piston motion which alternately compresses and expands working gas in the working space as a consequence of piston motion.
GAS BEARINGS. Because liquid lubricants can foul the heat exchangers or vaporize in the hot regions, Stirling engines are provided with a gas bearing lubrication system. Working gas is cyclically pumped into a gas bearing cavity 44 through a gas bearing inlet passage 46. Although the bearing cavity 44 appears in the drawing as two separate cavities 44A and 44B, the gas bearing cavity 44 is a continuous annular space within the piston. A check valve 48 permits the working space 8 pressure variations in the compression space 12 to pump working gas into the bearing cavity 44 but prevents gas flow in the opposite direction. The working gas within the cavity 44 flows out of the cavity 44 through multiple gas bearing pads 50. The gas bearing pads 50 are chambers that are spaced at annular intervals around the piston with flow restrictive passages into the gas bearing cavity 44. Consequently, the interfacing surfaces of the piston 28 and the cylinder 22 are lubricated, and the piston is centered, by the flow of the pressurized working gas from the gas bearing pads 50 into the small clearance gap between those interfacing surfaces and then into the working space 8 and the back space 43.
CENTERING SYSTEM. FPSEs typically have a net flow of gas over the cycle from the working space to the back space. One cause is that gas passage through the piston/cylinder clearance gap has a net flow in the out direction. The reason is that, although the volume of gas flow is the same in both directions, the density of gas flowing out of the workspace is larger than the density of gas flowing into the workspace. The density is larger because the pressure of gas in the workspace, when gas flows out of the workspace, is greater than the pressure of gas in the back space when gas flows out of the back space. More importantly, for machines with gas bearings, the bearings tend to pump gas out of the working space to the back space such as by the flow through the gas bearing cavity 44 and out the gas bearing pads 50. The reason is that the entire input of gas into the gas bearing cavity 44 is from the workspace 8, but the gas passing out the gas bearing pads 50 is divided between returning to the workspace and flowing to the back space 43. The cumulative effect of this preferential blow-by over many cycles is that the mean position of the piston creeps in. The mean position of a piston is the center or mid-point between the farthest excursions of the piston in opposite directions. The distance between the farthest opposite excursions of a point on the piston is the piston stroke and one half of the stroke is the piston amplitude of reciprocation.
The engine is provided with a centering system that compensates for this preferential blow-by and prevents the inward creep by the piston 28. The centering system illustrated in FIG. 1 includes a centering system piston passageway 52 (shown in dashed lines) extending from the inner end of the piston boss 36 and out through the sidewall 32 of the piston 28. The centering system also includes an annular groove 56 around the interior wall of the cylinder 22 that opens into the back space 43 through a centering cylinder passageway 54. Whenever the piston passageway 52 and the annular groove 56 come into registration, the centering system provides a gas conducting passageway between the back space 43 and the working space 8. They come into registration twice each cycle, once during each direction of travel of the piston 28. The engine is constructed so that they come into registration to permit gas flow between the back space 43 and the working space 8 when the piston is at or near it's designed mean position. More particularly, the passageway between the back space 43 and the working space 8 is opened whenever the piston is at a position that, if the piston were reciprocating around its designed mean position, the pressure difference between the pressure in the working space 8 and the pressure in the back space 43 at the two times of registration during each cycle would average zero. With zero average pressure difference there would be no net gas flow through the centering system during each cycle. However, if the mean piston position creeps in as a result of gas transfer from the working space 8 to the back space 43, then, at the position of registration, the averaged gas pressure in the back space 43, averaged over the two passings in registration, is greater than the averaged gas pressure in the working space 8 so there is a net gas flow from the back space to the working space. Consequently, if the piston mean position creeps in as a result of the preferential blow-by, gas will be returned from the back space 43 to the working space 8 whenever the gas passageway 52 is opened to the back space 43. Conversely, if the piston were to creep out as a result of transfer of working gas from the back space 43 to the working space 8, then, at the position of registration, the gas averaged pressure in the back space 43 is less than the pressure in the working space 8 so gas will be transferred back from the working space 8 to the back space 43.
Inherent Instability of a FPSE
Most free-piston Stirling engines that are designed according to prior art principles have a typical engine power curve that relates engine power to piston amplitude. FIG. 4 shows a typical power output curve but the scales will vary from machine to machine. Commonly, an FPSE drives an alternator that supplies electrical power to an electrical load although there are useful applications where the engine drives a mechanical load. The instability problem can be considered with regard to an electrical load but is also applicable to mechanical loads.
In the absence of the invention and the absence of a controller, engine power is an increasing exponential function of piston amplitude over the engine's operating range. Typically engine power increases as the square of the engine amplitude. That makes the engine unstable with a linear load, such as a resistive electrical load which varies with voltage squared. Those skilled in the art of Stirling engines are familiar with the typical power curve of FIG. 4.
Considering FIG. 4, if a power curve for a load on the FPSE does not have a greater slope than the power curve for the engine, the engine does not have a stable operating point. The displacer and piston amplitude of reciprocation progressively increase until the piston amplitude of reciprocation increases along the typical power curve beyond the physical stroke limit of the machine at which collision occurs. Because a resistive electrical load has a power curve that, like the engine power curve, varies exponentially as the square of voltage, the slope of the load's power curve does not exceed the slope of the engine's power curve. Consequently, the engine is not stable. This instability means that the engine will not operate around an operating point in response to load variations but instead engine stroke will increase and cause engine damage. This has been called the Achilles heel of the FPSE.
The prior art uses an engine controller to overcome this instability and for additional reasons. The engine controller is commonly interposed between the output of the engine's alternator and input of the ultimate electrical load. Therefore, the controller's input terminals are seen by the output of the engine's alternator as the engine's load. In normal operation the controller prevents the instability and runaway increase in piston and displacer amplitude of reciprocation. Unfortunately, there are occasions when a malfunction of the controller or a disconnection or shorting of a connection between the controller and the FPSE or its alternator causes the load seen by the FPSE to appear as an open circuit or as a short circuit. In either instance there is no load to consume engine power and therefore the conditions for runaway piston amplitude exist. The purpose and object of the invention is to provide simple mechanical modifications of the free-piston Stirling engine that prevent the above-described runaway increase of piston amplitude and engine power despite the occurrence of a malfunction of the type described above.